Co-precipitation synthesis and electrochemical properties of NASICON-type Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes

NASICON-type solid electrolytes have gathered wide attention to meet the requirements of high energy density Li-metal batteries. In this study, Li1.3Al0.3Ti1.7(PO4)3 (LATP) were synthesized through co-precipitation method. The composition, phase transformation, microstructure, and particle size of LATP powder were characterized in detail. Pure LATP phase can be obtained at 800 °C with primary particle size of ~ 590 nm. In addition, the ionic conductivities of LATP pellets increased at first and then decreased with the increased sintering temperature and time. Owing to the highest relative density (95.6%), LATP (sintered at 900 °C for 6 h, LATP-9006) exhibited a highest total conductivity of 2.19 × 10−4 S/cm with a low activation energy of 0.32 eV. The results suggest that co-precipitation synthesized LATP-9006 ceramic electrolyte could be a promising alternative toward achieving a safe solid-state battery. Moreover, the co-precipitation method is suitable for the preparation of other oxide ceramic electrolytes.


Introduction
Lithium-ion battery has become the standard power supply for mobile electronic devices such as electric vehicles, laptops, and mobile phones because of its high output voltage, high energy density, and small environmental pollution, etc. [1][2][3]. However, the polyolefin diaphragm with poor thermal dimensional stability and flammable and volatile organic liquid electrolytes restricted the development of lithium-ion battery toward large-scale and high-energy direction. In order to overcome the safety problem of liquid organic electrolyte, a lot of researches have been carried out on solid-state batteries (SSBs), which has become the focus of the next generation of battery system in comparison with conventional lithium-ion batteries [4]. Solid electrolytes (SEs) exhibit stable ion transport and wide electrochemical windows, thus promoting stable circulation of the battery system [5]. Benefiting from the excellent solid characteristic, the dendrite growth of the metal negative electrode and the generation of side reactions could be inhibited, which eliminates vaporization, flammability, and explosion of lithium-ion batteries with organic liquid electrolyte.
As a typical inorganic SEs with NASICON-type structure (rhombohedral unit, space group R3hc), Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) has attracted significant attention because of its high Li ? conductivity and excellent chemical stability to O 2 /H 2 O, so that material preparation and battery assembly can be carried out in an air environment, which further reduces manufacturing costs [6][7][8][9]. At present, there are many methods for successfully preparing LATP particles, such as melt quenching [10], sol-gel method [11], and solid-state reaction [12], etc. Among these methods, wet chemical synthesis method has received extensive attention in preparing uniform fine ceramic particles because of uniform reaction at the molecular level, which cannot only adjust the microstructure of the powder to improve the ion conductivity, but also effectively reduce the sintering temperature and reduce the total energy consumption of the crystallization process [13,14].
Co-precipitation method, as a typical wet chemical synthesis route, could effectively control the composition and morphology of the material at molecular level and prepare inorganic powders on a large scale in a short time and low temperature conditions [15], making it suitable for the preparation of ceramic electrolytes. Therefore, in this work, LATP solid electrolyte was prepared by co-precipitation method to confirm and expand the application of co-precipitation method in solid electrolytes. The phase transition process was conducted via thermogravimetrydifferential scanning calorimetry (TG-DSC), X-ray powder diffraction (XRD), and Fourier transform infrared (FTIR). Crystallized LATP powder is obtained at 800°C for 6 h with primary particle size of * 590 nm. In addition, densified LATP electrolytes were successfully fabricated at 900°C for 6 h with the co-precipitation-derived powder and their electrochemical properties were investigated systematically via electrochemical impedance spectroscopy (EIS).

Materials synthesis
LATP was synthesized by a modified co-precipitation method. Figure 1a shows the graphical illustration of the co-precipitation process. Initially, CH 3 COOLi and Al(NO 3 ) 3 Á9H 2 O were dissolved into a 100 mL deionized water with stirring at 80°C. NH 3 ÁH 2 O was added to keep the pH * 9.5 and the value was checked and kept steadily during precipitating. Afterward, 20 mL ethanol solution containing Ti(OC 4 H 9 ) 4 and 20 mL NH 4 H 2 PO 4 solution were introduced into the above solution to obtain white precipitate. The dispersion was stirred for another 2 h at 80°C. After vaporizing the solvent at 150°C for 1 day, the remained powder was transferred to a ceramic crucible and kept at different temperatures (500-1000°C, 6 h) to further determine the optimal calcination temperature. The resulting powder was then uniformly ground and pressed into disc-shaped pellets at 200 MPa and then sintered at different temperatures (800°C-1000°C) to investigate their sinter ability. To minimize volatilization of lithium, the pellets were buried in the sacrificial LATP powders. Finally, the samples were polished to a thickness of 1 mm and two blocking silver electrode layers coated on each side for electrochemical performance analysis.

Characterizations
The density of sintered ceramics was measured by Archimedes method. TG-DSC (STA449F5 Jupiter, Netzsch, Germany) and FTIR (Nicollet NEXUS670, USA) were performed to analyze the phase transition process. A Bruker D8 diffractometer XRD and Bet-tersize2600 laser particle size analyzer (Bettersize Instuments LTD. China) were carried out to investigate the phase structure and particle size distribution. The morphology and microstructure were recorded by scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100 F). EIS was conducted to evaluate the ionic conductivities of the LATP samples, employing two blocking silver electrode layers coated on each side (10 -1 -10 6 Hz, 5mV, CHI660E, Shanghai Chenhua Instrument Co., Ltd.).

Results and discussion
The scheme (Fig. 1a) presents the detailed steps for LATP particles obtained by co-precipitation method. During the whole co-precipitation process, NH 3 ÁH 2 O was added continuously to keep the pH * 9.5. At the same time, the temperature was kept at 80°C to ensure the rapid precipitation of metal cations (Al 3? , Ti 4? ). Finally, white LATP powder was obtained by drying and sintering. In order to determine the optimal calcination temperature, the precursor LATP powder was decomposed by TG-DSC. As shown in Fig. 1b, significant weight loss can be observed at 200°C , corresponding to the larger endothermic peak of DSC curve, which may be related to the volatilization of small molecule additives such as water and alcohol in precursor powder [16]. In addition, in the temperature range of 200-400°C, there is a weight loss platform corresponding to two exothermic peaks in DSC curve, which is mainly related to the decomposition of organic carbon in the precursor powder [17]. In the temperature range of 400-1000°C , only an obvious exothermic peak near 700°C was obtained, indicating that the precursor powder has undergone phase transformation at this temperature until stable LATP phase was formed, which further confirmed by the following XRD data. Figure 1c shows the XRD patterns of LATP powders obtained at different calcination temperatures (500-1000°C) to further determine the optimal calcination temperature. No diffraction peak could be found for the sample at 500°C, indicating that the powder is still in an amorphous state. When the calcination temperature rise to 600°C and 700°C, the Fig. 1 a Schematic preparation of LATP using a co-precipitation. b TG-DSC curves of the LATP precursors in the temperature range between 30°C and 1000°C with a heating rate of 10 K/ min. c XRD patterns of particles calcined at 500-1000°C for 6 h. d Size distribution and e SEM image of LATP particles calcined at 800°C sample showed a more obvious characteristic peak, and had a good matching degree with the pure LiTi 2 (PO 4 ) 3 (LTP) standard XRD pattern (PDF#35-0754) [18], indicating that LATP phase formed at this temperature range, which is consistent with the exothermic peak near 700°C in Fig. 1b. Meanwhile, the characteristic peak of LATP shifted to a large angle compared with LTP because of the ionic radius of Al 3? (0.54 Å ) was smaller than that of Ti 4? (0.60 Å ). Moreover, when the calcining temperature increased to 800°C, pure LATP was formed without impurity peaks, indicating that 800°C was the lowest temperature for obtaining pure LATP powder. AlPO 4 impurity peaks was observed at 900°C and 1000°C, which may be related to the volatile loss of lithium at high temperature [19,20]. Figure 1d and e showed the particle size analysis and SEM of LATP samples obtained at 800°C for 6 h to further analyze the particle state. The particle size of LATP powder is relatively uniform, and the particle size is mostly around 590nm (D 50 * 0.59 lm). The uniform and fine powders can effectively reduce the sintering temperature and increase the density to some extent, which is beneficial to obtain LATP ceramic electrolyte with excellent performance.
In order to further determine the structure of the powders, the FTIR results of samples at different temperatures (120-1000°C) were given in Fig. 2. The structure of the powder changes obviously before and after heat treatment. The samples prepared by co-precipitation (120°C, 6 h) showed clear transmission peaks at 1389 and 3143 cm -1 , corresponding to the N-H bands in NH 4? [21]. The powders preheated at different temperatures (500-1000°C) showed peak values at 734, 577, 645, 1625, and 1038 cm -1 . The peak at 1625 cm -1 is related to the absorption of H-O-H bond of water on the sample surface [22]. The peak at 1038 cm -1 is attributed to the asymmetric and symmetric stretching vibration of P-O band in PO 4 tetrahedron [23]. Due to the Ti-O stretching vibration of TiO 6 octahedron, there are several intensity peaks near 577-734 cm -1 [24,25]. In addition, NH 4? characteristic peaks gradually disappeared at 800°C, and other characteristic peaks were sharp and visible in the range of 577-734 cm -1 ; meanwhile, with calcining temperature increasing, no obvious changes could be found in the structure peaks, indicating the crystal orientation of LATP is gradually formed after calcination at 800°C [25,26], which is consistent with the results of XRD diffraction pattern in Fig. 1c. On this account, FTIR spectra can also screen the crystallinity of the samples to determine the phase structure changes of the powders.
The TEM and HRTEM images were carried out to further identify the phase composition and morphology of the LATP particles obtained by co-precipitation. As shown in Fig. 3a, the LATP particles have an irregular morphology, being in good agreement with the SEM results (Fig. 1e). Figure 3b and c showed the enlarged (b) and (c) regions drawn in the white frame in Fig. 3a, respectively. The HRTEM images clearly showed that the LATP grains were surrounded by amorphous regions. A similar phenomenon existed in Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 electrolyte [17]. The interplanar spacings of nanocrystalline grains of 0.42, 0.36, and 0.60 nm correspond to (104), (113), and (012) lattice planes of LATP, respectively. The selected area electron diffraction pattern (inset of Fig. 3b) indicated a well-defined crystal structure for these regions of the particles. The above results indicated that the LATP obtained at 800°C is partially crystalline, while crystalline nanoparticles and amorphous regions coexist. The EDX spectrum (Fig. 3d) further confirmed the elements of Al, Ti, O, and P; meanwhile, as shown in the EDS of the LATP particle ( Fig. 3e1-e4), the Al, Ti, O, and P elements dispersed in the nanosheets homogeneously, which further confirmed that Al 3? replaced Ti 4? to synthesize LATP electrolyte powder successfully. Figure 4a showed the XRD patterns of LATP pellets sintered at 800°C, 850°C, 900°C, 950°C, and 1000°C for 6 h, respectively. The characteristic peaks of all samples are well corresponding to those of pure LTP (PDF#35-0754), indicating that all samples have NASICON-type structure. The increased sintering temperature is helpful to improve the crystallinity of LATP ceramics. However, the AlPO 4 peaks began to appear at 900°C, and the intensity of the diffraction peak became more obvious with the increase of sintering temperature. When the temperature rises to 1000°C, the characteristic peak intensity of LATP is weakened, which indicates that too high sintering temperature seems to be harmful to the crystallinity of LATP. Figure 4c-f showed the surface SEM images of LATP ceramic obtained at different sintering temperatures. It can be seen that with the increase of sintering temperature, the grain size first increased and then tended to be constant. It is reported that AlPO 4 phases are mostly aggregated at the grain boundaries of LATP, which could limit the growth of grains obtained at 900-1000°C [27]. The increased grain size reduced the number of grain boundaries and suppressed the occurrence of defects, thus increased the density and electrochemical performance of LATP ceramics [17,28]. Moreover, the inset of Fig. 4b-f showed the cross-section SEM images for LATP pellets. The LATP-900 6 was less pores and vacancies (Fig. 4d), while other samples appeared to be rough and filled with noticeable pores. It also suggested the relative density of LATP-900 6 can be increased compared with that of other samples. Relative densities (q rd ) of LATP electrolytes at different sintering temperatures were shown in Fig. 4; Table 1. The LATP-900 6 exhibited the highest relative density (95.6%). A small amount of grain boundary AlPO 4 phase at 900°C fills the grain boundary gap and improves the density of the ceramics [17,29]. However, with the sintering temperature further increasing, the q rd value decreased to 95.4 and 92.3% for LATP sample sintered at 950°C and 1000°C, Fig. 3 Morphologies, microstructures, and element ratio of the LATP particles synthesized by a coprecipitation: a TEM, b, c HRTEM images, inset of (b) electronic diffraction pattern of a selected area, d EDX spectrum, and e elemental mappings for Al, Ti, P, and O elements respectively. This performance degradation could be related to the excessive formation of secondary AlPO 4 phases across the grain boundaries, thereby inducing severe microcracks through the lattice shrinkage of AlPO 4 , and then leading to a decrease of the relative density [28].
For obtaining LATP ceramic electrolytes with excellent performance, control of sintering time is another crucial parameter. Figure 5a showed the XRD patterns of LATP ceramic sintered at 900°C for different holding times (3-12 h, denoted as LATP-900 3 , LATP-900 6 , LATP-900 9 , and LATP-900 12 , respectively). It can be seen that with the increase of sintering time, the intensity of LATP diffraction peak first increased and then decreased, accompanied by the enhanced AlPO 4 impurity peaks, indicating that long-term sintering will also cause the appearance of impurities, which is similar to the situation observed with the increase of sintering temperature. Figure 5be showed the surface SEM images of LATP ceramic obtained at different sintering times. No obvious grain changes could be found with the increase of sintering time, but the LATP-900 6 sample had the highest relative density (95.6%, as shown in Fig. 5f; Table 1). It is also proved that reasonable sintering temperature and holding time are important parameters for increasing the density of LATP ceramics.
The impedance spectra of the LATP ceramics obtained at different sintering temperature was displayed in Fig. 6a. All plots of the samples consisted of two depressed semicircles at high to medium frequencies and a tail at the low frequency region. The  Table 1 The value of bulk conductivity (r b ), total conductivity (r t ) measured at 25°C, and relative density (q rd ) for Li 1 left semicircle in the high-frequency range can represent grain boundary resistance (R gb ) and the left intercept of the semicircle were assigned to the bulk resistance (R b ) [30]. Another semicircle that is overlapped with the right side of the grain boundary semicircle may represent interfacial resistance (R int ), and the intercepts of the semicircle at the low frequency sides are ascribed to total (R b ? R gb ? R int ) impedances. The straight line in the low frequency range corresponds to the blocking electrodes, i.e., Ag, on both sides of LATP pellets, which are typical features of ionic conductive ceramics [31,32]. As shown in Fig. 6a, a steeper slope in the low frequency region and a smallest semicircle diameter in the high    Fig. 6b displayed a typical Nyquist diagram of the LATP-900 6 impedance spectrum, and derived the corresponding circuit elements by fitting the experimental data points to the circuit shown in the inset. The curve fits the experimental data very well, indicating that the equivalent circuit is suitable to the samples. Among them, Wo is the Warburg diffusion contribution. CPE gb and CPE int are their associated capacitances [18,30,33]. The values of R b , R gb , and R int were 68 X, 138 X, and 293 X, respectively. The interfacial contribution to the total resistance [R int / (R b ? R gb ? R int )] was * 58%, which is much higher than that of R b and R gb , indicating that the interfacial resistance is responsible for the total resistance of the electrolyte. This phenomenon may be related to the irregular LATP surface as shown in Figs. 4d and 5c. Generally, the ionic conductivity can be calculated by the equation: r = L/R 9 S. Where r, R, L, and S represent ionic conductivity, resistance, thickness, and area of LATP ceramic electrolyte, respectively. Through data fitting and calculation, the total ionic conductivity (r t ) and bulk ionic conductivity (r b ) of the LATP samples increased at first, and then decreased with the increase of sintering temperature ( Fig. 6c; Table 1). Meanwhile, compared with the impedance spectra of the samples at different sintering time (Fig. 6d), LATP-900 6 sample exhibited a maximum r t value of 2.19 9 10 -4 S/cm with a relatively high r b value (1.54 9 10 -3 S/cm), which is much higher than that of other samples. This result could be attributed to the increased relative density shown in Figs. 4 and 5. The decreased r t value of the samples at high sintering temperature or long sintering time could be associated with the decreased relative density. In addition, as an impurity with extremely low ionic conductivity, AlPO 4 will hinder the grain boundary lithium-ion transmission process and thus reduce the grain boundary ionic conductivity [17,29].
The temperature dependence of impedance was measured to get the activation energy (E a ) of LATP-900 6 (Fig. 7a) in the test temperature range from 25 to 80°C. Furthermore, the relationship between the E a and r t of the LATP-900 6 pellets was calculated from Eq. (1) [17]: where k B is Boltzmann constant, T and A are the ambient temperature and given parameter, respectively. The E a of Li-ion conduction in LATP-900 6 is 0.32 eV. This result is comparable to that of other reports (Table 2), indicating the higher Li-ion mobility of LATP-900 6 sample, which is beneficial to the high performance of solid electrolyte in a wide temperature range [34]. Table 2 summarizes the ion conductivities and activation energy of LATP electrolyte prepared via different methods [7, 8, 10-13, 25, 35-37]. It is clear that co-precipitation synthesis of LATP-900 6 could deliver an ion conductivity of 2.19 9 10 -4 S/cm at 25°C, which is comparable to that reported by many other groups, indicating the LATP-900 6 could prove a most advantageous in terms of electrochemical performance.

Conclusion
In summary, NASICON-type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) ceramic electrolytes were successfully fabricated using co-precipitation method. The phase transition and microstructure of LATP precursor were analyzed by TG-DSC, XRD, SEM, and HRTEM. The results showed that the pure LATP phase could be obtained at 800°C, and the crystal structure was surrounded by amorphous regions. The LATP-900 6 electrolyte shows the highest relative density (95.6%), thereby enabling the electrolytes exhibited a high ionic conductivity of 2.19 9 10 -4 S/cm at 25°C with a low activation energy of 0.32 eV. It is anticipated that the co-precipitation synthesis is an effective way to improve the electrochemical properties of ceramic electrolyte, such as ionic conductivity and compatibility with electrode, which will promote the application of solid electrolytes in solid-state batteries.

Declarations
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.